What causes a stencil portal mask leaking outside its window?

The mask writer and the masked geometry use mismatched stencil reference values or comparison functions, so the masked objects pass the stencil test in pixels the mask never marked.

How do I catch this when it only happens for some players?

Capture it automatically from the player's device with the full stack trace, the device and OS, the build, and a breadcrumb trail of what they did. That turns a vague complaint into a specific, reproducible issue you can fix without owning the hardware it happens on.

How to Fix a Stencil Portal Mask Showing Geometry Outside the Window in Unity

Quick answer: Have the mask quad write a stencil ref with Comp Always and Pass Replace, and have portal contents render with Comp Equal against the same ref so they only appear where the mask wrote.

Stencil portals work by writing a tag into the stencil buffer on a mask shape, then only drawing the portal world where that tag exists. A leak means the read pass is not actually testing against the written value. Here is how to fix it.

How to fix it

1. Write the mask first

On the mask quad's shader use Stencil { Ref 1 Comp Always Pass Replace } and disable color writes with ColorMask 0 so it only stamps the stencil buffer.

2. Test in the masked pass

On the portal interior materials use Stencil { Ref 1 Comp Equal } so fragments are discarded wherever the stencil value is not 1, confining them to the window.

3. Control render order with queues

Put the mask in a queue earlier than the contents (e.g. mask at Geometry-1, contents at Geometry) so the stencil is written before the masked objects test it.

Catching the ones you can't reproduce

The hardest version of this to fix is the one you can't reproduce — it only happens on a player's hardware, OS, driver, or save state, under conditions that simply aren't present on your machine. A report that says “it crashed” or “it froze” gives you nothing to act on, so the bug survives release after release while quietly costing you players.

Automatic error capture closes that gap. Each failure arrives with its full stack trace, the device and OS, the build number, and a breadcrumb trail of what the player did right before it broke, so even a failure you have never seen becomes a specific, reproducible issue. Fold identical failures into one signature ranked by how many players each hits, and your worklist sorts itself worst-first instead of arriving as a stream of vague complaints.

This is where a tool like Bugnet earns its place. Its SDK captures every Unity error automatically with the full stack trace plus device, OS, memory, build, and game-state context, folds duplicates into one grouped issue with an occurrence count, and ties each to the build it first appeared on — so you fix the problem that hurts the most players first and confirm it is gone when its signature disappears from the next release.

The bug you can't reproduce isn't gone — it's just invisible until you capture it from the player's device.